1. Field of the Invention
This invention relates to apparatus for flameless atomization of a sample for atomic absorption analysis and, more particularly, to such an apparatus which comprises a tubular body in which the sample is contained and heated by means of an electric current passed through the body. Such apparatus are commonly referred to as "graphite furnaces" with allusion to the fact that the tubular body, hereinafter frequently called the sample tube, is normally fabricated of graphite.
2. Summary of the Prior Art
Apparatus of the type to which the invention relates customarily employ a cylindrical graphite sample tube having a radial bore at substantially the midpoint of its length, the bore providing a port through which the sample may be introduced into the tube. The electrical heating current is passed through the tube by means of respective, generally annular, electrodes, typically also of graphite, in pressure contact with, and supporting, the tube ends. The annular configuration of the electrodes permits a beam of radiation, of selected spectral characteristics, to be directed through the tube to effect analysis of the atomized sample in a manner well known in the art of atomic absorption spectroscopy. In this connection, reference may be had to U.S. Pat. No. 2,847,899. Normally, atomization of the sample requires heating of the tube (usually performed in stages to effect drying and ashing of the sample preparatory to its atomization) to extremely high temperatures; consequently, cooling jackets of high thermal conductivity material are provided for the electrodes and the tube is enveloped in a mantle of flowing protective gas to prevent its combustion. This requires defining an annular chamber for the gas surrounding the tube while still enabling access to the sample port.
In prior art, graphite furnaces of the type described in U.S. Pat. No. 3,778,156, the sample tube is surrounded for the greater part of its length directly by the cooling jackets, the proximal ends of which have axial collars overlapping so as to form an annular axial gap. The cooling jackets, in turn, are enclosed by a cylindrical housing forming an annular space surrounding the proximal ends of the cooling jackets. Protective gas introduced into this annular space flows through the annular axial gap between the cooling jacket collars into an annular space between the cooling jackets and graphite tube and from there through the sample port into the interior of the graphite tube. Such a structure does not provide ready accessibility of the sample port although one sample tube known in the prior art has an inspection arrangement, designed in the manner of a pinhole camera, screwed into the housing in alignment with the sample port of the graphite tube. This observing arrangement can be unscrewed so that a relatively large diameter opening remains in the housing through which the sample port of the graphite tube is accessible. Thus, it is possible to guide a pipette through the opening of the housing to the sample port in order to introduce the sample. Handling, however, is painstaking and difficult since sample injection requires unscrewing of the observing arrangement and the radial distance between the sample port and the opening in the housing is quite large.
It is also known in the prior art to provide a slide closure member in the housing between the cooling jackets, by which a housing opening is optionally exposed or covered. Such an arrangement also requires separate opening and closing actions when introducing the sample and creates the need to ensure that the opening is closed during heating of the tube. Moreover, in this case also, the radial distance between the housing opening and the sample port in the graphite tube is undesirably great thus complicating the injection of the sample (by means of a pipette, for example).
Another shortcoming of prior art graphite furnaces in which the sample tube is substantially surrounded by the metallic cooling jackets is the fact that undesired temperature changes sometimes occur in the sample tube even though the heating current is maintained at a constant level, for the following reason.
As long as the cooling jacket parts facing the sample tube are bare, they reflect the heat radiated outwardly by the heated graphite tube. However, in the course of operation, the bare cooling jacket parts become coated with graphite dust originating from the hot graphite tube so that an increasing portion of the radiant heat energy emitted by the sample tube is absorbed and carried off by the coolant flowing through cooling jackets. Therefore, when the surfaces of the cooling jackets are bare, the temperature of the sample tube at a given current level is greater than when the surfaces are blackened by graphite dust. In the prior graphite furnaces, the temperature of the sample tube also depends on the temperature of the cooling jackets and thus, for instance, on the intensity of cooling.
In the graphite furnace shown in the aforementioned U.S. Pat. No. 3,778,156, an inert gas stream enters through the sample port in the mid-length region of the graphite tube and from there flows to both ends of the graphite tube. In the vicinity of the ends of the tube, there are provided further radial bores. A protective gas stream flowing over the exterior of the graphite tube enters the graphite tube through these radial bores thus precluding admission of air which would result in conbustion of the tube at high operating temperatures. These protective gas streams also flush the atomized sample from the graphite tube, the vapors passing over the surface of the electrodes and the exposed portions of the contact surfaces by which cooling jackets abut the electrodes. The atomized sample may contain corrosive vapors which attack exposed metallic surfaces in particular the cooling jacket surfaces.
In a co-pending U.S. Pat. application Ser. No. 453,114 filed Mar. 20, 1974 and assigned to the same assignee as the present invention, there is disclosed a graphite furnace in which a protective gas stream flows from the ends of the sample tube inwardly and exits through the sample port. The graphite tube is surrounded by an annular protective gas chamber defined by a cylindrical outer housing wall, cooling jackets, the inner end surfaces of the electrodes and the cylindrical outer surface of the graphite tube; this chamber is in communication with the sample port and is connected with a protective gas outlet. In such a furnace, an additional protective gas stream can be introduced into the annular chamber via a protective gas inlet formed by a nipple extending radially through the housing wall and into the annular chamber terminating the close proximity to the sample port. This arrangement prevents the atomized sample from emerging through the sample port and passing into the annular chamber to form deposits there at the cool wall portions, in particular, the cooling jackets. Injection of the sample is effective either via this nipple or by way of an annular slide closure rotatable with the nipple to expose an opening in the housing.